1. Field of the Invention
This invention relates to a process of preparing a powder of metal hydroxides, wherein the powder is intended for use in alkaline storage batteries.
2. Background Art
In general, when using powders in industrial processes, it is desirable that these powders be composed of particles having a high apparent density, a centered granulometry and good flowability, wherein the latter may be quantified by measuring the angle of repose. The apparent density is defined in Metals Handbook, Vol. 7, p. 274 (1984), The American Society for Metals. The measurement of the angle of repose is described in Metals Handbook, Vol. 7, p. 282 (1984). When these three characteristics can be obtained, they minimize resulting problems of heterogeneity and reproducibility when filling a specific capacity, and they also minimize the formation of dust particles.
When the application involves powders of hydroxides or metal salts, the most widespread manufacturing process employed to produce these characteristics is granulation, atomization being a typical example. This process, which in other respects is highly efficient, requires the use of organic adjuvants with a high carbon content to bond the fine particles to each other. Unfortunately, the presence of these adjuvants results in powders which are unsuitable for use in alkaline storage batteries, because the decomposition of the carbon chains produces carbonates, resulting in a deterioration of the electrical performance of the batteries.
To prepare the nickel hydroxide which is employed in these batteries, the usual process is to briefly bring into contact a solution of a nickel salt and a base, for example, soda (sodium hydroxide). The product obtained is in the form of very fine particles which are produced as the result of an extremely intensive nucleation process generated by strong supersaturation. The nickel hydroxide is difficult to separate from the reaction medium, and due to the fine size of the particles, it is necessary to perform long filtration processes before it can be dried and then ground to produce a powder. Moreover, this powder does not display the three characteristics described above: a high apparent density, controlled granulometry, and flowability.
Also known, from Japanese patent publication JP-A 63 91120, is a process for preparing a nickel hydroxide powder comprising the steps of introducing into a reactor a nickel salt, a strong base, and an ammonium salt, the temperature of the reaction being maintained between 20.degree. C. and 80.degree. C. and the pH value within the reaction being held between 9 and 12.
The present applicants have discovered that, if the process is performed as described in this document, the nickel hydroxide is produced:
(a) either in the form of irregularly shaped particles that must be subsequently ground, resulting in a powder of which the density of the grains is too small; PA1 (b) or in the form of nearly spherical particles of 15 to 20 .mu.m, but having a high internal porosity, resulting again in a grain density that is too small. PA1 simultaneously and in a continuous manner introducing into a reactor: PA1 maintaining the temperature of the reactor at a value greater than about 80.degree. C.; PA1 stirring the mixture in the reactor with a propeller turning at 300 to 1,000 revolutions per minute; and PA1 filtering, washing, and drying the particles of precipitate thus obtained.
Furthermore, the described hydroxide cannot be used as such in an electric cell, for it does not contain a specified number of additives needed for good functioning.